1. Field of the Invention
The present invention relates generally to surface cleaning and, more particularly, to a method and apparatus for megasonic cleaning of a semiconductor substrate following fabrication processes.
2. Description of the Related Art
Megasonic cleaning is widely used in semiconductor manufacturing operations and can be employed in a batch cleaning process or a single wafer cleaning process. For a batch cleaning process, the vibrations of a megasonic transducer creates sonic pressure waves in the liquid of the cleaning tank which contains a batch of semiconductor substrates. A single wafer megasonic cleaning process uses a relatively small transducer above a rotating wafer, wherein the transducer is scanned across the wafer, or in the case of full immersion a single wafer tank system is used. In each case, the main particle removal mechanisms with megasonic cleaning are due to cavitation and acoustic streaming. Cavitation is the rapid forming and collapsing of microscopic bubbles generated from dissolved gas when sonic energy is applied to a liquid medium. Upon collapse, the bubbles release energy which assists in particle removal through breaking the various adhesion forces which adhere the particle to the substrate. Acoustic streaming is the fluid motion induced by the acoustic wave through the fluid when RF power is applied to a piezoelectric transducer.
FIG. 1A is a schematic diagram of a batch megasonic cleaning system. Tank 100 is filled with a cleaning solution. Wafer holder 102 includes a batch of wafers to be cleaned. Transducer 104 creates pressure waves through sonic energy with frequencies near 1 Megahertz (MHz). These pressure waves, in concert with the appropriate chemistry to control particle re-adhesion, provide the cleaning action. Because of the long cleaning time required for batch cleaning systems, as well as chemical usage, efforts have been focused on single wafer cleaning systems in order to decrease chemical usage, increase wafer-to-wafer control, and decrease defects in accordance with the International Technology Roadmap for Semiconductors (ITRS) requirements. Batch systems suffer from another disadvantage in that the delivery of megasonic energy to the multiple wafers in the tank is non-uniform and can result in ‘hot spots’ due to constructive interference, or ‘cold spots’ due to destructive interference, each being caused by reflection of the megasonic waves from both the multiple wafers and from the megasonic tank. Constructive interference can cause damage to sensitive features or pattern on the wafer substrate, and thus the average energy must be lowered to ensure any hot-spots are below the damage threshold. In cases of cold spots, insufficient cleaning occurs, and therefore, a higher megasonic energy must be applied in order to reach all regions of the wafers in wafer holder 102. In either case, a compromise must be reached to minimize damage while still providing high enough average energy to enable cleaning.
FIG. 1B is a schematic diagram of a single wafer cleaning tank. Here, tank 106 is filled with a cleaning solution. Wafer 110, supported by carrier 108, is submerged in the cleaning solution of tank 106. Transducer 104 supplies the energy to clean wafer 110. The cleaning solutions are typically designed to modify the zeta potential between the surfaces of the wafer and a particle removed from the surface of the wafer through the acoustic energy supplied by transducer 104 to prevent particle re-attachment. The cleaning solution concentration should be maintained within a fairly tight range in order to maintain a suitable zeta potential between the surfaces. However, for features such as lines, contacts, spaces, vias, etc., defined on a surface of the substrate, the particle may redeposit on the surface of the substrate due to the inability to maintain a specific cleaning solution concentration, i.e., replenish the cleaning solution, at the particle-substrate interface within the region defined by the feature. Further, in the case of a transducer oriented perpendicular to the substrate surface (as in a single-tank megasonic system), high-aspect ratio features may shadow, or shield, the lower regions of the feature from megasonic energy and cavitation. For a transducer oriented parallel to the wafer surface, cavitation can occur within the feature, but acoustic streaming is not in the most favorable direction to facilitate moving a detached particle away from the substrate.
Additionally, electrodepostion operations, in particular electroless plating, is commonly used for the deposition of a film on a substrate. For example, a copper film may be deposited on a substrate through electroless plating. One of the shortcomings of electroless plating is that the presence of any bubble formation in the features of a patterned substrate undergoing electroless plating will lead to voids in subsequent plating operations. Another shortcoming of electroless plating into high aspect ratio features is mass transport of the fresh reactants from the solution into the features, and mass transport of byproducts out of the same features.
In view of the foregoing, there is a need for a method and apparatus to provide a single wafer megasonic cleaning configuration that is capable of replenishing cleaning chemistry into regions defined by the feature in order to prevent a particle removed by the acoustic energy from re-depositing on the wafer. In addition, there is a need for controlling bubble formation in the vicinity of features undergoing electroless plating operations and improving mass transport of reactants and by-products into and out of high aspect ratio features.